1. Field of the Invention
The present invention generally relates to an aqueous polyurethane resin dispersion suitable for use as an aqueous adhesive, and relates to the use thereof as an adhesive.
2. Description of Related Art
When bonding bases such as rubber, leather, metal, glass, plastic such as polyvinyl chloride (PVC), foam, fiber and paper using an aqueous polyurethane-based solution or dispersion, the adhesive strength and heat resistance as well as the coatability of adhesives are generally important.
A base coated with an adhesive of an aqueous dispersion is laminated, for example, by a wet lamination method of applying an adhesive on a base and laminating with contact bonding without removing moisture, or a dry lamination method of drying to remove moisture and laminating in a state where the base has adhesiveness. In a case in which an adhesive is applied on a base, which is smooth and is impermeable, such as metal, plastic, or glass, characteristics such as permeability are not required to produce adhesiveness. However, since a rubber or leather, as the base, subjected to a buffing treatment for the purpose of improving its adhesiveness has unevenness and voids on the surface, the permeability of the adhesive is required. Furthermore, higher adhesive strength is expected of porous bases such as fibers and papers by means of an anchoring effect utilizing the permeability of the adhesive.
In the case in which the adhesive is applied manually using a brush or applied mechanically using a knife coater, good spreadability of the adhesive, excellent leveling properties required to form a coating layer having a uniform thickness, and properties such as permeability required for porous bases such as those of fiber are deemed to be important to stably obtain a high adhesive strength. Furthermore, in the case of contact bonding where an adhesive is applied on two bases and the adhesive surfaces are laminated to each other, good mobility of polymer molecules in the adhesive is deemed to be important.
To generate the adhesiveness by removing moisture, a so-called reactivation step of drying under the conditions of about 50 to 70° C. is required and thus requiring the ability to melt at relatively low temperatures within a short time.
Therefore, there has hitherto been a great need to provide an adhesive which is superior in coatability regardless of the surface condition (smoothness or unevenness), non-porosity (impermeability) or porosity (permeability) of bases of, for example, rubber, plastic, leather, and fiber in the bonding operation, and which also exhibits good permeability on the porous base and is superior in adhesive strength and heat resistance.
As such an adhesive, a process for preparing an aqueous dispersion or solution of polyurethane using a prepolymer containing an ethylene oxide unit as a nonionic hydrophilic moiety in a polyether chain is proposed (Japanese Unexamined Patent Application, First Publication No. Sho 53-26897).
However, the polyurethane resin of the aqueous dispersion has a particle diameter of larger than 0.3 μm and a uniform aqueous dispersion is hardly obtained when the content of the ethylene oxide unit is small, while the viscosity becomes higher and the adhesive strength on the base tends to decrease when the content increases. Since the surface tension is large within the range (content) where sufficient adhesive strength can be obtained, the resulting adhesive did not have excellent leveling, permeability, and adhesiveness sufficient to use in bonding of buffed rubbers, leathers, and fibers having characteristics such as porosity.
U.S. Pat. No. 5,432,228 describes a process for preparing an aqueous solution or dispersion of a polyisocyanate polyadduct having at least one of a carboxylate group and sulfonate group as a hydrophilic group. The adhesive prepared from the dispersion contains the polyurethane resin having a small particle diameter but substantially contains no surfactant and, therefore, there arose problems in that the surface tension is large, similar to those in Japanese Unexamined Patent Application, First Publication No. Sho 53-26897, and in that the spreadability upon coating, and performances such as permeability on the porous base and adhesive strength are not satisfactory.